It is conventional design practice to use a hydrokinetic torque converter in an automotive vehicle driveline between an internal combustion engine and multiple-ratio gearing. The torque flow path through the gearing is interrupted when a forward clutch or a reverse clutch for the gearing is released.
If the friction clutches of the transmission remain applied when the vehicle engine is operated at zero throttle (i.e., the engine idles), the idle state causes the hydrokinetic torque converter to function as a neutral clutch as the turbine of the converter is stationary and the impeller of the converter is driven at engine idle speed. This loading of the torque converter creates a hydrokinetic parasitic loss that degrades driveline efficiency.
The present invention is capable of being used in a multiple-ratio gearing system of the kind shown, for example, in U.S. Pat. No. 5,389,046. Such transmissions typically do not have the ability to achieve a neutral idle state since the forward and reverse clutches are applied with a relatively high line pressure, even with the engine idling. With the engine throttle closed and the engine idling, the line pressure for such conventional control systems may be 50 psi, for example. The forward drive clutch or the reverse drive clutch thus is fully engaged when the engine throttle is closed.
Attempts have been made to cause the clutch to be operated in a continuously slipping state during neutral idle. An example of a transmission control system intended to function in this fashion is described in U.S. Pat. No. 5,272,630, which discloses a neutral idle control for unloading the converter to eliminate undesirable noise, vibration and harshness in the driveline when the engine is idling and the transmission is in the drive mode. Another transmission control system with a neutral idle feature is described in copending patent application Ser. No. 09/410,918, filed Oct. 1, 1999. That copending patent application is assigned to the assignee of the present invention. These prior disclosures are incorporated herein by reference, and reference to them may be made to supplement the present disclosure.
It is necessary in a control system with a neutral idle feature to complete the clutch engagement relatively quickly following termination of the neutral idle mode, but this must be done with minimal driveline disturbance. It is necessary as well to avoid excessive slipping during clutch engagement in order to maintain clutch durability. A short engagement time is desirable to achieve maximum clutch durability, but smoothness in the engagement suggests that a long engagement time should be used.
It is possible to apply a slipping clutch rapidly by using an electronic controller that uses a closed-loop control method based upon turbine speed and impeller speed as input variables. Another possible control method of this type may use clutch pressure that is incremented during clutch engagement based on estimated converter torque. Such control methods, however, are not capable of providing smooth clutch application at all engine throttle settings within a desired, short engagement time.